1. Field of the Invention
This invention relates generally to an instrument using room temperature sensors that measure magnetic susceptibility variations in the body, and more particularly to such an instrument employing an improved water-bag technique to eliminate background tissue response.
2. Discussion of the Related Art
Millions of people suffer from diseases related to the metabolism of iron in the human body. Among these are Cooley's anemia (also known as thalassemia), sickle cell anemia, and hemochromatosis. Magnetic susceptibility measurements are an important non-invasive technique for measuring iron stores in the liver.
The need to obtain liver iron measurements is especially acute in the case of Cooley's anemia, or thalassemia. In this disease, where the blood is deficient in hemoglobin, patients must undergo blood transfusions in order to survive. These blood transfusions must be frequent (every 2 to 4 weeks). However, the repeated transfusions create a chronic iron overload with an abnormal buildup of iron in the liver, spleen, and heart. Sickle cell anemic patients undergoing frequent blood transfusions also suffer from liver iron overload. There are other conditions which affect liver iron concentration leading to the need for accurate, frequent, non-invasive measurements of iron in the liver and other areas of the body. This iron overload must be removed continually by chelation therapy, and iron stores must be monitored regularly to maintain the desired levels.
Biomagnetic susceptometry is a diagnostic procedure that involves noninvasive, radiation-free, direct, and accurate, measurement of the magnetic susceptibility of organs and tissue within a human or animal body. Biomagnetic susceptometry can be used to measure human iron stores contained in the liver.
Some existing instruments for such measurements are based on Superconducting Quantum Interference Devices (SQUIDs). However, they tend to be complex and expensive. SQUIDs based on High-Temperature Superconductors (HTS) could, in principle, reduce the cost of biomagnetic susceptometry. However, even at liquid-nitrogen temperatures, the operating costs would be higher than those of ordinary instruments operating at room temperature.
Presently available biomagnetic susceptometers have drawbacks in several different technical areas, as discussed below.
A key problem in the susceptometric liver iron measurement is the background signal produced by the magnetic susceptibility of the patient's body tissues. This tissue background signal can be many times larger than that due to iron in the liver, and it varies according to the shape of the patient's body. This variability can easily mask the magnetic susceptibility signal due to liver iron. To eliminate this background tissue response, the common practice is to put a water-bag between the sensor unit and the patient's body. See Farrell et al., Magnetic Measurement of Human Iron Stores, IEEE Transactions on Magnetics, Vol. Mag. 16, No. 5, pp. 818–823 (September 1980).
It is useful to first describe the conventional water-bag method and discuss some of its important limitations. The biomagnetic liver-iron measurement uses a sensor unit comprising a magnetic-field sensor and a coil that produces a magnetic field. When this sensor unit is sitting by itself in empty space, the magnetic sensor sees only the applied magnetic field from the coil. When the sensor unit is placed next to the patient's abdomen, the body tissues become slightly magnetized by the applied magnetic field, producing a small change in the magnetic field at the magnetic sensor. This change in magnetic field includes a contribution due to iron in the liver, plus a contribution from the magnetic susceptibility of the body tissues themselves.
The conventional water-bag method eliminates most of the error due to the susceptibility response of the body tissues. This method takes advantage of the fact that most body tissues have magnetic susceptibilities close to that of water. In existing biomagnetic susceptometer systems, the water-bag method works as shown in FIGS. 9A and 9B. Water-bag 91 is in the form of a flexible bellows which surrounds the lower end of sensor unit 92. Initially, the bellows is compressed as the patient's abdomen 93 is pressed up against the sensor unit. Then, the patient is moved down, away from the sensor unit (arrow 94), using a special non-magnetic, pneumatic table. As the patient is lowered, the bellows is filled with water, so that the magnetic susceptibility signal from the body tissues is replaced by an equivalent signal from the water in the water-bag. Hence, the magnetic sensor sees no net change in magnetic field due to the response of the body tissues. However, the iron in the liver has a susceptibility different from that of water. This difference in susceptibility produces a magnetic-field signal which changes as the patient's abdomen moves farther from the sensing unit.
This method has some disadvantages. First, in prior water-bag systems a special mechanism is required to add or withdraw water form the water-bag, as needed to maintain constant pressure. Second, noise may be introduced into the magnetic susceptibility measurement because of variations in the way the water-bag fills. Additionally, the need to fill and empty the water-bag makes it difficult to make rapid changes in the distance between the sensor and the patient. This limitation is not a problem with existing low temperature biomagnetic susceptometers, which use extremely stable sensors operating at liquid-helium temperatures. However, in a room-temperature instrument, the patient-sensor distance must be modulated continuously, at a frequency near 1 Hz, in order to cancel out the effects of temperature drift in the applied-field coils and magnetic sensors. It would be very difficult to fill and empty a water-bag at this rate. The conventional water-bag method also makes it difficult to scan the magnetic susceptometer along the body, in order to map out susceptibility variations within the body. This scanning capability is potentially useful in the liver iron measurement, as a means of determining the possible susceptibility response of tissues surrounding the liver, such as the lungs. Scanning measurements are also potentially useful in other applications such as the detection of ferromagnetic foreign bodies in a host.
An important issue in a room-temperature biomagnetic susceptometer is to minimize the noise caused by various things such as temperature drift and motion, among others, in the sensors used to detect the susceptibility response of the body.